Motion transmitting devices using various combinations of concentrically and eccentrically mounted and rotatable means to provide speed differentials between shafts are known in the art. Many use an internal toothed gear and a meshing eccentric gear, the teeth of which mesh with a portion of the teeth of the internal toothed gear when the eccentrically mounted gear is caused to orbit. In many of these devices a relatively few teeth carry load which limits the torque capacity of the device. Because of the sliding motion of the teeth, friction losses and heat build-up occur causing wear and a reduction in the useful life of the device. Others use an eccentrically mounted gear which meshes with fixed pins arranged in a cylindrical array, the major axis of which is concentric with a shaft carrying an eccentric for the gear. In this device there is sliding motion between the fixed pins and the gear teeth causing increased friction, the build-up of heat and reduced life of the device. Also, few elements carry the load, resulting in a low torque carrying device.
In a more recent development, the teeth of an eccentric external gear mesh with the pins of a chain, the chain following a wobbling path as the gear orbits. The gear has at least one less tooth than the chain has pins. The chain remains generally in one location, its movement being limited by openings into which the chain pins extend, the openings being located in capturing plates which may define at least part of the housing of the device. The eccentric motion is translated into rotary motion at the output of the device, as is the case with the other discussed devices. One advantage in the last described device is the torque carrying tooth-pin engagement through an arc of approximately 90.degree..